We report measurements of photo-luminescence from GaAs/AlGaAs heterojunctions in the ultra-quantum limit. New features are observed for low electron densities at high magnetic field, which we suggest are associated with the formation of negatively charged excitons, both singlet and triplet states. The dependence of the spectra on temperature and carrier density are presented in support of this theory. (C) 1998 Elsevier Science B.V. All rights reserved.